This invention relates generally to a method and apparatus for measuring mechanical properties of a monomolecular film, and, more particularly to a method and apparatus for measuring shear modulus and viscosity of a monomolecular film (monolayer).
The principle property studied of monolayers has been the surface pressure, .pi., as a function of the molecular area density. Surface pressure is defined as the difference between the surface tension of pure water and that of the monolayer covered water and is usually displayed as a function of the area per molecule, i.e. the .pi.-A diagram. The .pi.-A diagram has many characteristics of a P-V diagram in three dimensions and the observed features have, by analogy, been assigned phase labels similar to those in three dimensions.
In addition to the .pi.-A diagram, measurements of surface potential and surface viscosity have been made for some monolayers. Surface viscosity has been measured by rotational or channel viscometers. Rotational viscometers used in the past lacked sensitivity and acted as an obstruction to material flow during compression. Channel viscometers are unreliable because the pressure is not hydrostatic for films that can support a shear. Prior to the present invention, no techniques have been available that are capable of determining shear modulus. Shear modulus is an important parameter in that it distinguishes a liquid from a solid: only a solid can support a shear.
Many substances have .pi.-A diagrams with a kink which has been interpreted to mean a phase change from liquid to solid. Correlation of shear modulus measurements with the .pi.-A diagrams will provide useful information on the properties of monolayers.
Therefore, it is an object of the present invention to provide a method and apparatus for measuring shear modulus of a monomolecular film.
It is also an object of the present invention to provide an improved method and apparatus for measuring viscosity of a monomolecular film.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.